Days and nights are equal
During the Autumn Equinox, the Sun rises due east and sets due west, no matter where on the planet you are. Days and nights are just about equal. Autumn marks the lengthening nights and darkening skies. Trees change colors as the leaves fall to the ground. The energy in the air has shifted from the extroverted, free-spirited nature of summer fun to a quieter, more reflexive foreshadowing of winter.
Prepare for the cold winter months ahead
In song, Autumn refers to the mature stage of life just past the prime, but before the retirement years. The lightheartedness of summer has passed, the harvest is almost in, school has begun. There is knowledge to be gained. The motivation to work is much greater at this time as folks instinctively prepare for the cold winter months ahead.
The transitory nature of life
This is a time to save resources. Stock up on foods for the long fallow period ahead. Preserve the foods you harvest, make jams and prepare lots of meals to freeze for unbearably cold days ahead that will keep you indoors. Bring your winter wear out of the back of the closet. As autumn progresses and the nights grow longer still, you may find yourself in a more reflective state. Holidays such as Halloween and the Day of the Dead remind us of the transitory nature of life. Crops die, waiting for the chance to be reborn in the next cycle.